In an optical communications network, an optical signal is modulated with digital information and transmitted from a source location to a destination location. Many individually modulated “optical frequency channels” at different optical frequencies can be combined for transmission in a single optical fiber, whereby the transmission capacity of the optical fiber increases many times. The modulated optical frequency channels can be simultaneously amplified by a single optical amplifier. A chain of optical amplifiers, connected to each other with spans of optical fiber, allows for massive data transmission over very long distances. The digital data encoded in each of the optical frequency channels can be transmitted over hundreds and even thousands of kilometers.
When a transmitter at a particular optical frequency fails, the optical frequency channel “disappears”, leading to a disruption of transmission of digital information encoded in that channel. When the transmitter's optical frequency shifts, reception of neighboring channels may be affected. When an optical modulator of a particular optical frequency channel fails, the modulation is lost, and no digital information is transmitted to a destination. To be able to detect these impairments, center optical frequency, modulation bandwidth, and optical power of each and every frequency channel need to be closely monitored.
One type of optical channel monitors of the prior art is based on optical filters having a narrow transmission band, that can be scanned across the spectrum of an optical signal in the network. Center frequency (or wavelength) of the transmission band of the filter is continuously scanned, while the optical signal at the output of the filter is measured at the same time. Optical frequency channels appear as peaks on the resulting scanned spectrum of the optical signal. By measuring position, width, and height of the peaks, the center optical frequency, the modulation bandwidth, and the optical power of each optical frequency channel can be determined. Disadvantageously, scanning optical channel monitors tend to have a slow time response, in particular when optical performance of one optical frequency channel needs to be evaluated or monitored in real time. This occurs because the whole spectrum needs to be scanned to obtain current information on the optical frequency channel of interest.
Parallel detection of light at different optical frequencies can be used to overcome the slow response drawback. Referring to FIG. 1, a prior-art detector-array optical channel monitor 100 includes an input port 102, an input collimator 104, a diffraction grating 106, an output collimator 108, and a detector array 110 connected to a controller 112. An input optical signal 101 coupled to the input port 102 is collimated by the input collimator 104, dispersed into individual optical frequency channels 105 by the diffraction grating 106, and is focused by the output collimator 108 onto the detector array 110. All optical frequency channels 105 are thus detected simultaneously. The controller 112 processes data to obtain the channel information such as power, wavelength, and spectral width.
Modern photodetector arrays capable of detecting light in the optical telecommunication wavelength region, typically at wavelengths of about 1.54 microns, can provide up to 4-5 pixels per optical frequency channel. Unfortunately, this is often not enough to perform reliable and unambiguous spectral measurements. Due to non-linearity of angular dispersion of the diffraction grating 106, position of individual pixels of the detector array 110 is usually not correlated with respect to individual optical frequency channels 105. This could be overcome by constructing a detector array with a custom, varying pixel pitch, but custom designs of pixel arrays can be prohibitively expensive.